The skin is made up of three layers including the epidermis, dermis and subcutaneous or fat layer. The epidermis of the skin is the outer layer and is a continuously renewing tissue. The majority of cells in the epidermis are keratinocytes that originate in the basal layer. The keratinocytes migrate from the basal layer to the outermost layer known as the stratum corneum.
The epidermis provides a protective barrier against transcutaneous water loss. The epidermis also prevents most bacteria, viruses and other foreign substances from entering the body. The epidermis also protects the internal body from trauma.
The dermis is a layer of fibrous and elastic tissue that provides the skin flexibility and strength. The underlying fat layer provides insulation from heat and cold. The fat layer also provides an energy depot.
When skin ages, the epidermal and dermal layers atrophy. It has been reported in studies that aged skin is characterized by a thinning epidermis. (Gambichler et al., 2006, J. Dermatological Sci. 44:145-152). It has also been reported that the epidermal turnover rate slows with aging. (Baumann, 2007, J. Pathology 211:241-251). As a result, a protracted rate of stratum corneum replacement occurs. Moreover, epidermal atrophy and slower wound healing can also occur. Often, less effective desquamation is also prevalent. Such decelerated cell turnover can cause the skin surface to appear rough and dull in appearance. In sum, as skin ages it becomes dry, wrinkled and fragile, and a loss of skin barrier function results.
Photoaging superimposes the effect of chronic ultraviolet induced (UV-induced) damage in addition to normal intrinsic skin aging, which leads to further changes in the skin. It has been reported that epidermal atrophy may occur in some individuals. (Yaar et al., 2007, Br. J. Dermatology 157:874-887).
In addition to skin aging, treatment using glucocorticoid hormones may inhibit keratinocyte proliferation. However, chronic glucocorticoid hormone treatment is accompanied by side effects including reduced epidermal thickness, decreased number of keratinocytes, and loss of skin barrier function. (Chebotaev et al., 2007, J. Investigative Dermatology 127:2749-2758).